At the present time, telecommunication systems are largely based on fiber optic cables. For example, optical networks based on fiber optic cables are currently utilized to transport Internet traffic and traditional telephony information. In such applications, it is frequently necessary to provide an optical signal over significant distances (e.g., hundreds of kilometers). As optical signals travel through the optical fibers, a portion of their power is transferred to the fiber, scattered, or otherwise lost. Over appreciable distances, the optical signals become significantly attenuated. To address the attenuation, optical signals are amplified. Typical optical amplifiers include rare earth doped amplifiers (e.g., Erbium-doped fiber amplifiers).
Raman amplifiers may be utilized. A Raman amplifier relies upon the Raman scattering effect. The Raman scattering effect is a process in which light is frequency downshifted in a material. The frequency downshift results from a nonlinear interaction between light and the material. The difference in frequency between the input light and the frequency downshifted light is referred to as the Stokes shift which in silica fibers is of the order 13 THz.
When photons of two different wavelengths are present in an optical fiber, Raman scattering effect can be stimulated. This process is referred to as stimulated Raman scattering (SRS). In the SRS process, longer wavelength photons stimulate shorter wavelength photons to experience a Raman scattering event. The shorter wavelength photons are destroyed and longer wavelength photons, identical to the longer wavelength photons present initially, are created. The excess energy is conserved as an optical phonon (a lattice vibration). This process results in an increase in the number of longer wavelength photons and is referred to as Raman gain.
The probability that a Raman scattering event will occur is dependent on the intensity of the light as well as the wavelength separation between the two photons. The interaction between two optical waves due to SRS is governed by the following set of coupled equations:                     ⅆ                  I          P                            ⅆ        z              =                                        λ            S                                λ            P                          ⁢                  g          R                ⁢                  I          S                ⁢                  I          P                    -                        α          P                ⁢                  I          P                                        ⅆ                  I          S                            ⅆ        z              =                            g          R                ⁢                  I          S                ⁢                  I          P                    -                        α          S                ⁢                  I          S                    where Is is the intensity of the signal light (longer wavelength), Ip is the intensity of the pump light (shorter wavelength), gR is the Raman gain coefficient, λs is the signal wavelength, λp is the pump wavelength, and αs and αp are the fiber attenuation coefficients at the signal and pump wavelengths respectively. The Raman gain coefficient, gR, is dependent on the wavelength difference (λs−λp) as is well known in the art.
As is well understood in the art, SRS is useful for generating optical gain. Optical amplifiers based on Raman gain are viewed as promising technology for amplification of WDM and DWDM telecommunication signals transmitted on optical fibers. Until recently, Raman amplifiers have not attracted much commercial interest because significant optical gain requires approximately one watt of optical pump power. Lasers capable of producing these powers at the wavelengths appropriate for Raman amplifiers have come into existence only over the past few years. These advances have renewed interest in Raman amplifiers.
Existing high power lasers dispose individual discrete lasers in, for example, 14 pin butterfly packages. The output beams from the individual devices are either polarization division multiplexed or wavelength division multiplexed into a single beam. To the extent that these systems multiplex more beams, the systems are able to generate a higher power output beam. In addition, WDM and DWDM telecommunications systems operate over large bandwidth ranges and require broad wavelength pump lasers to effectively use Raman amplification. These multiplexing schemes address this by operating at multiple wavelengths. However, these lasers become quite cumbersome and costly when the number of butterfly packages exceeds a relatively small number. Accordingly, the power that that can be achieved cost-effectively is limited.
Another type of high power laser is referred to as an incoherently beam combined (IBC) laser. An example of a known IBC laser is described in U.S. Pat. No. 6,208,679. Known IBC lasers utilize a dispersive external cavity and various optics to selectively provide feedback to emitters of a unitary emitter array. The selective feedback causes emitters of the unitary emitter to laser across a relatively broad, although limited, spectrum. Additionally, the dispersive external cavity and optics multiplex output beams from emitters of the unitary emitter array.